How do you test an oxygen sensor with a volt meter?

This takes a little effort but it can be done with a volt meter. Typically they measure this with the repair computers in the dealerships and such. For a single wire sensor trace the wire color back into the cars computer. The computer is usually under a seat, passenger side dash or under the console. To make sure you traced the right wire disconnect the o2 sensor and using the connector end (of the wiring harness NOT the o2 sensor)use your meter and "ohm out" the wire ie check connectivity. On the computer side the harness connector will hopefully be the typical molex connectors that you can press your probe into from the outside. (Do not disconnect from the computer it wont start!).

Now that you know which wire into the computer comes from the o2 sensor, reconnect the o2 sensor and warm up your car. You may consider leaving the voltmeter probe in the molex connector before starting if you are concerned about poking around after it is running. In the volts setting (0-2 or 0-5 volts) leave the positive lead in the connector and the negetive lead to ground (usually anything metal connected to the frame). When you first start your car your numbers will be low 0.1-0.3 volts as this is a rich mixture necessary until the car warms up. Once warm expect numbers in the mid to high range. meaning 0.5 to 0.8 volts. If you bounce the throttle you will see a cyclic response from low to high 0.3 then 0.8 or so as the engine emits its exhaust. Consult your specific auto manual for acceptable ranges.

It is usually between 0 an 1 volt with 0 = rich and 1 equal to lean. Your auto manual should give you outputs for idle, and possibly under load (driving at a constant rpm)

If you have a multi wire sensor the same applies but you have to fish out the "signal" wire coming from the sensor. The other wires are typically for heating the sensor.

... I have done it myself on several cars but I don't know what they all look like and behave like so use this only as one data point.